Publications and other reference materials referred to herein are incorporated herein by reference in their entirety and are numerically referenced in the following text and respectively grouped in the appended Bibliography, which immediately precedes the claims.
Many techniques exist for particle size and concentration analysis (PSA), they can be reviewed for reference in the book by Terry Alan (1) “Introduction to Particle Size Analysis”. The most commonly used techniques are optical, based on the interaction of the measured particles with laser radiation. Especially when approaching the particle size range around 1 micron and below, most of these techniques suffer from inaccuracies due to the effect of the real and imaginary part of the particle's refractive index. It is known, for example, that in some techniques, such as techniques based on Fraunhoffer diffraction analysis, light absorbing particles would be over sized due to energy loss resulting from the absorption, while in high concentration, particles would be under sized due to secondary scattering etc.
An optical technique that is less sensitive to these problems is known as Time of Transition or TOT. In this technique the interaction of a scanning, focused laser beam and the particles is analyzed in the time domain rather than in the intensity domain, resulting in lower sensitivity to variation in the refractive index. A detailed description of the technique appears in the paper (2) by Bruce Weiner, Walter Tscharnuter, and Nir Karasikov. To a great extent, in this technique, a de-convolution algorithm, of the known laser beam profile, from the interaction signal, derives the size. The concentration is derived from the number of interactions per unit time within the known volume of the focused laser beam.
The interaction of the particles in the TOT technique is with a focused scanning laser beam. In order to measure smaller particles, a smaller focused spot should be used. However according to diffraction laws for a Gaussian laser beam, if the beam's waist is D, the divergence of the beam is proportional to λ/D where λ is the laser's wavelength. The trade-off between the ability to resolve small particles, to the focus volume and the accuracy in measuring concentration is obvious. Thus if the TOT technique is targeted to resolve and measure particles in the micron and sub-micron range it would be limited in its ability to measure low concentrations as the instantaneous focus volume is small and the interaction rate of particles is low. On the other hand, taking a larger spot will improve the concentration measurement rate but will degrade the quality and resolution of the size analysis.
An improvement could be achieved by using a shorter wavelength. This could have a limited effect of, as high as, a factor of 2 only, since going to too short a wavelength will result in absorption of the laser light by the optics and, in the case of particles in liquid, also absorption by the liquid.
It is therefore the purpose of the present invention to introduce a new technique and means to decouple between the two contradicting requirements: the ability to resolve small particles and the ability to measure low concentration using measurements based on single particle interactions.
Further purposes and advantages of this invention will appear as the description proceeds.